As electronic components get smaller, lighter and more sophisticated, the semiconductor packaging form is changing from wire bonding type to flip-chip type.
A flip-chip semiconductor apparatus has a configuration in which electrode portions on a substrate are connected to a semiconductor device via bump electrodes. The thus configured semiconductor apparatus suffers from the following problem: When heat is applied as in temperature cycling, the bump electrodes are stressed due to a difference in coefficient of thermal expansion between the substrate made of an organic material such as epoxy resin and the semiconductor device, thus causing the bump electrodes to have defects such as cracking. To suppress the occurrence of such defects, it is widely performed to seal gaps between the semiconductor device and the substrate with a liquid sealing material called “underfill” to secure them to each other, thereby improving the thermal cycle resistance.
A liquid sealing material for use as the underfill is required to be excellent in injection properties, adhesive properties, curing properties, storage stability and the like, and not to generate voids. In addition, a portion sealed with the liquid sealing material is required to be excellent in moisture resistance, thermal cycle resistance, reflow resistance, cracking resistance, warpage resistance and the like.
To satisfy the foregoing requirements, an epoxy resin-based material is widely used as the liquid sealing material for use as the underfill.
It is known that controlling the difference in coefficient of thermal expansion between a substrate made of an organic material such as epoxy resin and a semiconductor device and reinforcing bump electrodes by adding a filling material (hereinafter referred to as “filler”) made of an inorganic substance such as a silica filler to a liquid sealing material are effective to improve the moisture resistance and the thermal cycle resistance, in particular the thermal cycle resistance of a portion sealed with the liquid sealing material (see Patent Literature 1).
In flip-chip semiconductor apparatuses, with miniaturization of low-K layers and increasing use of lead-free solder bumps and Cu pillars in recent years, a liquid sealing material is required to have much lower thermal expansion (i.e., have a lower coefficient of thermal expansion) in order to prevent a low-K layer from being broken or solder bumps to be cracked due to thermal stress.
On the other hand, gaps between semiconductor devices and substrates and distances between adjacent bumps tend to be smaller.
To achieve lower thermal expansion of an underfill, a high degree of filling (i.e., a high filling rate) of a filler is essential; however, the viscosity of the underfill increases with increasing filling rate of the filler, which leads to poor injection properties of the underfill in injection into gaps between a semiconductor device and a substrate.
Besides, when a silica filler is used, the silica filler has on its surface a number of silanol groups which are hydrophilic groups, therefore does not have a good compatibility with hydrophobic components (e.g., epoxy resin constituting a base compound) of an underfill, and thus tends to exhibit poor filler dispersibility in an underfill.
To achieve a high degree of filling of a filler, a finer filler is to be used; however, such a finer filler leads to the increase in its surface area in an exponential function manner. As a result, dispersibility of the filler deteriorates because of the foregoing reason, which causes the viscosity of an underfill to increase, resulting in poor injection properties of the underfill in injection into gaps between a semiconductor device and a substrate.
Meanwhile, to increase the inorganic filler content, a method in which an inorganic filler is surface-treated with a silane coupling agent is known (for example, see Patent Literature 2).